Definition
A switch setting for the standby battery system that places the standby battery on standby — not actively powering the buses, but ready to automatically connect and supply essential instruments if the main electrical system fails or bus voltage drops below a set threshold.
Plain English
A switch position that says to the standby battery: 'Stay ready. If the main electrical system fails, take over automatically.'
Context Anchor
Seen on standby battery controls during preflight setup and when managing an electrical-system malfunction in flight.
Derivation
From 'arm,' meaning to make a system ready to act — the same sense used in 'arming' a missile, an alarm, or an autopilot mode. The system is not yet doing its job, but it is primed to do so the moment conditions call for it.
Why Pilots Care
Maintains power to attitude, navigation, and communication instruments during total electrical failure, reducing risk of spatial disorientation.
Analogy
It is like arming a home alarm: the alarm is ready to act if a door opens, but it is not sounding just because it is armed.
Intuition Check
ARM does not mean the battery is already powering the airplane. Here, ARM means the standby battery system is enabled and ready to take over automatically if normal power fails.
Example Sentence 1
Before takeoff, confirm the standby battery switch is in the ARM position so backup power is available if the main electrical system fails.
Example Sentence 2
When the alternator failed in clouds, the ARM position allowed the standby battery to power the instruments without interruption.